A sealed secondary battery such as a non-aqueous electrolyte secondary battery represented by a high capacity lithium ion secondary battery is widely used as a driving power source for a portable device or the like or a storage battery for a household or a vehicle.
FIG. 8 is a sectional view schematically illustrating a configuration of a sealed secondary battery. In the sealed secondary battery, electrode group 4 formed by laminating or winding positive electrode plate 1 and negative electrode plate 2 via separator 3 is accommodated in battery case 5 together with an electrolyte and an opening portion of battery case 5 has a sealed structure which is sealed with sealing plate 10 via gasket 6. Positive electrode tab 11 introduced from one electrode plate (for example, positive electrode plate) of electrode group 4 is joined to sealing plate 10 also serving as one external terminal and a negative electrode tab (not illustrated in FIG. 8) led out from the other negative electrode plate of electrode group 4 is joined to battery case 5 also serving as the other external terminal.
As a method of the related art which welds by irradiating a battery case made of a metal such as iron or SUS and a negative electrode tab made of nickel or copper with a laser, there is a method which welds by irradiating the battery case and the negative electrode tab with the laser from an outside of battery case 5 (for example, see Japanese Patent Unexamined Publication No. 2004-158318). FIG. 9 and FIG. 10 are views illustrating welding of battery case bottom portion 13 and negative electrode tab 14 of battery case 5 of the related art described in Japanese Patent Unexamined Publication No. 2004-158318.
In FIG. 9, electrode group 4 formed by spirally winding a positive electrode plate and a negative electrode plate via a separator is inserted into cylindrical battery case 5, and negative electrode tab 14 welded to the negative electrode plate overlaps bottom portion 13 of battery case 5 at a central portion of battery case 5, negative electrode tab 14 is in contact with bottom portion 13 of battery case 5 by contact rod 12, pulse laser 7 is emitted from the outside of battery case 5, and thus battery case bottom portion 13 and a portion of negative electrode tab 14 of battery case 5 are welded.
In the configuration disclosed in Japanese Patent Unexamined Publication No. 2004-158318, pulse laser 7 is emitted from the outside of battery case 5, while the temperature of the tab surface is measured, a plurality of pulse irradiations are continued until the signal thereof exceeds a predetermined threshold value.
Although a method of emitting pulse laser 7 from negative electrode tab 14 inside battery case 5 is also conceivable, there is a possibility that pulse laser 7 hits and burns electrode group 4 and when a by-product such as spatter and debris which is generated at the time of welding remains inside battery case 5, it causes a short-circuit failure, so it is considered that the method of emitting pulse laser 7 from the outside of battery case 5 described in Japanese Patent Unexamined Publication No. 2004-158318 is preferable.
FIG. 10 is a sectional view illustrating a joining portion between battery case bottom portion 13 and negative electrode tab 14 of battery case 5 of FIG. 9 and melting proceeds from the surface of battery case bottom portion 13 by emitting pulse laser 7 from battery case bottom portion 13 side, welded portion 15 having a linear welding trace reaches the joining surface with negative electrode tab 14 as the irradiation time elapses, further, the laser irradiation is stopped in a state where welded portion 15 further proceeds to the inside of negative electrode tab 14, and thus case bottom portion 13 and negative electrode tab 14 of battery case 5 are joined to each other. Generally, in welding by the pulse laser, since a plurality of irradiations are continued at the same place, the heat of the laser concentrates on one point, and this heat is transferred by heat conduction into the material, the welding area of welded portion 15 of battery case bottom portion 13 on the irradiation side of pulse laser 7 increases.
In addition, as a method of the related art which welds by irradiating a sealing plate made of a metal such as aluminum and a positive electrode tab made of aluminum or the like with a laser, there is method which welds the other end of the tab to the sealing plate by the laser by abutting the other end of the tab against the sealing plate and emitting from the tab side while continuously scanning the other end of the tab with a fiber laser beam having a spot diameter less than the thickness of the tab (for example, see Japanese Patent No. 4647707).
In the configuration disclosed in Japanese Patent No. 4647707, deep penetration type welding (keyhole welding) is performed with a smaller spot diameter using a fiber laser.
The keyhole welding will be described in detail with reference to FIG. 11. In a case where the power density of the laser is high, the surface of the laser irradiation portion is heated to the evaporation temperature or higher, the surface is recessed by reaction force during evaporation and thus a deep keyhole is formed. In the keyhole, the laser is absorbed by the Fresnel (absorption by multiple reflections) at the keyhole inner wall surface (generally, front wall surface in traveling direction) or the bottom portion, and metal vapor (plume) is generated and is ejected from the keyhole opening. The penetration depth of the keyhole type melt welded portion is determined by the depth of the keyhole and molten metal flow from a tip of the keyhole. Here, a portion where the molten metal flow convects while the laser beam is traveling is called a molten pool. In this molten pool, heat is diffused by heat conduction to the surrounding material and heat convection into the atmosphere, and the molten pool gradually cools and solidifies.
FIG. 12 and FIG. 13 are views illustrating welding of sealing plate 10 and positive electrode tab 11 of the related art described in Japanese Patent No. 4647707. FIG. 12 is a plan view illustrating a method of the related art described in Japanese Patent No. 4647707 of laser welding positive electrode tab 11 to sealing plate 10. In addition, FIG. 13 is a view illustrating a cross section of a welded portion including positive electrode tab 11 and sealing plate 10.
In FIG. 13, the end portion of positive electrode tab 11 led out from the electrode group is continuously scanned with a continuous oscillating laser 16 having a spot diameter less than the thickness of positive electrode tab 11 along the width direction of positive electrode tab 11 in a state of being abutted against sealing plate 10 and thus welded portion 15 is formed and the end portion of positive electrode tab 11 is welded to sealing plate 10 by the laser.